Double-pipe heat exchanger and refrigeration cycle system

ABSTRACT

In a double-pipe heat exchanger, a groove non-forming range is set as a non-groove surface in each of, in an inner surface of a heat transfer area increasing pipe, an inner surface of a part of the heat transfer area increasing pipe, which is held in close contact with an inner surface of an outer pipe, and a part of the inner surface of the outer pipe, which defines a second flow path in cooperation with an outer surface of the heat transfer area increasing pipe.

TECHNICAL FIELD

The present invention relates to a double-pipe heat exchanger in which two flow paths are formed by combining circular pipes having different pipe diameters, and to a refrigeration cycle system using the double-pipe heat exchanger.

BACKGROUND ART

In the double-pipe heat exchanger, a circular pipe having a small diameter (hereinafter described as an inner pipe) is inserted to a circular pipe having a large diameter (hereinafter described as an outer pipe). An inside of the inner pipe is defined as a first flow path, and a part on an outer side of the inner pipe and an inner side of the outer pipe is defined as a second flow path so that heat is exchanged between a first fluid inside the first flow path and a second fluid inside the second flow path.

Further, as an effort to enhance heat transfer performance in such a double-pipe heat exchanger, a structure disclosed in, for example, Patent Literature 1 is known. That is, in Patent Literature 1, there is proposed a method of enhancing the heat transfer performance due to an effect of an increase in heat transfer area, which is obtained by inserting a heat transfer area increasing pipe having a multilobed lateral cross section to an inside of an annular second flow path defined between the outer side of the cylindrical inner pipe and the inner side of the cylindrical outer pipe.

CITATION LIST Patent Literature

[PTL 1] JP 2012-063067 A

SUMMARY OF INVENTION Technical Problem

Patent Literature 1 merely discloses the effort to increase the heat transfer area. Hence, the inventors of the present invention focus on suitably transferring heat when the heat is exchanged in a two-phase refrigerant.

The present invention has been made in view of the above, and it is therefore an object thereof to provide a double-pipe heat exchanger capable of enhancing heat exchange performance when a two-phase flow flows in a second flow path, or the like.

Solution to Problem

In order to achieve the above-mentioned object, according to one embodiment of the present invention, there is provided a double-pipe heat exchanger, including: an outer pipe; an inner pipe inserted to an inner side of the outer pipe, the inner pipe forming an annular region between the outer pipe and the inner pipe, and forming a first flow path in an inner side thereof; and a heat transfer area increasing pipe arranged on the inner side of the outer pipe and an outer side of the inner pipe, the heat transfer area increasing pipe having projections and depressions in a radial direction, and forming a second flow path in the annular region, in which a groove non-forming range is set in each of, in an inner surface of the heat transfer area increasing pipe, an inner surface of a part of the heat transfer area increasing pipe, which is held in close contact with an inner surface of the outer pipe, and a part of the inner surface of the outer pipe, which defines the second flow path in cooperation with an outer surface of the heat transfer area increasing pipe, and the groove non-forming range includes a non-groove surface, in which a groove forming candidate range includes a part excluding the groove non-forming range from a part of the inner surface of the heat transfer area increasing pipe, which defines the second flow path in cooperation with the outer surface of the inner pipe, a part of the outer surface of the heat transfer area increasing pipe, which defines the second flow path in cooperation with the inner surface of the outer pipe, and a part of an outer surface of the inner pipe, which defines the second flow path in cooperation with the inner surface of the heat transfer area increasing pipe, and in which grooves extending along a flow direction are formed in at least part or an entirety of the groove forming candidate range.

Advantageous Effects of Invention

According to the one embodiment of the present invention, it is possible to enhance the heat exchange performance when the two-phase flow flows in the second flow path.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view for illustrating an internal structure of a double-pipe heat exchanger according to a first embodiment of the present invention in a direction orthogonal to a pipe axis.

FIG. 2 is a sectional view of the double-pipe heat exchanger taken along the line II-II of FIG. 1.

FIG. 3 is a view for illustrating a second flow path of FIG. 2 in an enlarged manner.

FIG. 4 is a view for illustrating a part of FIG. 3, in which an outer pipe, a heat transfer area increasing pipe, and an inner pipe are separated from each other for the sake of illustration.

FIG. 5 is a view for illustrating Example 1 of the refrigeration cycle system using the double-pipe heat exchanger.

FIG. 6 is a view for illustrating Example 2 of the refrigeration cycle system using the double-pipe heat exchanger.

FIG. 7 is a view for illustrating Example 3 of the refrigeration cycle system using the double-pipe heat exchanger.

FIG. 8 is a view for illustrating Example 4 of the refrigeration cycle system using the double-pipe heat exchanger.

FIG. 9 is a view according to a second embodiment of the present invention, for illustrating in the same manner as in FIG. 3.

FIG. 10 is a view according to a third embodiment of the present invention, for illustrating in the same manner as in FIG. 3.

DESCRIPTION OF EMBODIMENTS

Now, embodiments of the present invention are described with reference to the accompanying drawings. Note that, in the drawings, the same reference symbols indicate the same or corresponding parts.

First Embodiment

FIG. 1 is a view for illustrating an internal structure of a double-pipe heat exchanger according to a first embodiment of the present invention in a direction orthogonal to a pipe axis. FIG. 2 is a sectional view of the double-pipe heat exchanger taken along the line II-II of FIG. 1. Note that, for the sake of clarity of illustration, the illustration of a heat transfer area increasing pipe described later is omitted in FIG. 1. A double-pipe heat exchanger 1 has a double pipe structure in which an inner pipe 5, which is a circular pipe having a relatively small diameter, is concentrically inserted to an inner side of an outer pipe 3, which is a circular pipe having a relatively large diameter. An inner space of the inner pipe 5 functions as a first flow path 7. On the other hand, a heat transfer area increasing pipe 11 is accommodated in an annular region 9 on an outer side of the inner pipe 5 and the inner side of the outer pipe 3.

The heat transfer area increasing pipe 11 has a plurality of projecting portions 13 and a plurality of depressed portions 15 as relative projections and depressions in a radial direction. As illustrated in a lateral cross section of FIG. 2, the plurality of projecting portions 13 are radially formed to project toward a radially outer side of the heat transfer area increasing pipe 11. Further, the plurality of projecting portions 13 are arranged at substantially equal intervals in a circumferential direction. On the other hand, the plurality of depressed portions 15 are each positioned between a corresponding pair of the projecting portions 13 in the circumferential direction. Those depressed portions 15 are also positioned at substantially equal intervals in the circumferential direction. Therefore, when viewing the entire heat transfer area increasing pipe 11, the plurality of projecting portions 13 and the plurality of depressed portions 15 are alternately positioned in the circumferential direction.

In the present invention, various modes are conceivable as a projection shape of the projecting portion and a depression shape of the depressed portion when viewed in the lateral cross section of FIG. 2 for illustrating the heat transfer area increasing pipe. As an example, the first embodiment is as follows. The heat transfer area increasing pipe 11 includes a plurality of outer close-contact portions 17, a plurality of inner close-contact portions 19, and a plurality of continuous portions 21. As illustrated in FIG. 2, outer surfaces 17 a of the outer close-contact portions 17 of the heat transfer area increasing pipe 11 and an inner surface 3 b of the outer pipe 3 are held in close contact with each other. In particular, in this embodiment, the outer surface 17 a and the inner surface 3 b are held in surface contact with each other. That is, the outer surface 17 a of the outer close-contact portion 17 of the heat transfer area increasing pipe 11 has substantially the same curvature as that of the inner surface 3 b of the outer pipe 3. Similarly, inner surfaces 19 b of the inner close-contact portions 19 of the heat transfer area increasing pipe 11 and an outer surface 5 a of the inner pipe 5 are held in close contact with each other. In particular, in this embodiment, the inner surface 19 b and the outer surface 5 a are held in surface contact with each other. That is, the inner surface 19 b of the inner close-contact portion 19 of the heat transfer area increasing pipe 11 has substantially the same curvature as that of the outer surface 5 a of the inner pipe 5. Note that, this state of having the same curvature may be obtained in a separated state of each of the outer pipe 3, the inner pipe 5, and the heat transfer area increasing pipe 11, or may be obtained in a state at the completion of an assembly process that involves application of any force from a center side of the double-pipe heat exchanger 1 or an outer side thereof in the radial direction.

The continuous portions 21 are each positioned between the adjacent outer close-contact portion 17 and inner close-contact portion 19. In this embodiment, the plurality of outer close-contact portions 17 are positioned at equal intervals in the circumferential direction. The plurality of inner close-contact portions 19 are also positioned at equal intervals in the circumferential direction. When viewing the entire heat transfer area increasing pipe 11, an arrangement mode in the order of the outer close-contact portion 17, the continuous portion 21, the inner close-contact portion 19, and the continuous portion 21 is repeated in the circumferential direction. Note that, the projecting portion 13 and the depressed portion 15 do not have a definite boundary. The projecting portion 13 is formed by the outer close-contact portion 17 and parts of the continuous portions 21, which are closer to the outer side in the radial direction. The depressed portion 15 is formed by the inner close-contact portion 19 and parts of the continuous portions 21, which are closer to an inner side in the radial direction.

In the above-mentioned annular region 9, an inner side of the projecting portion 13 and an outer side of the depressed portion 15 each function as a second flow path 23. That is, the second flow path 23 is defined in the annular region 9 by the heat transfer area increasing pipe 11.

More specifically, the second flow path 23 has parts in two modes. A first-mode part is defined by an inner surface 17 b of the outer close-contact portion 17, inner surfaces 21 b of a corresponding pair of the continuous portions 21, and the outer surface 5 a of the inner pipe 5. Further, a second-mode part is defined by an outer surface 19 a of the inner close-contact portion 19, outer surfaces 21 a of a corresponding pair of the continuous portions 21, and the inner surface 3 b of the outer pipe 3. The first-mode parts and the second-mode parts are alternately arrayed in the circumferential direction.

In such a configuration, a first fluid flows in the first flow path 7, and a second fluid flows in the second flow path 23. The first fluid and the second fluid have different temperatures, and heat is exchanged between the first fluid and the second fluid through thermal conduction between the inner pipe 5 and the heat transfer area increasing pipe 11.

In general, a heat exchange amount Q, a heat transfer area A, a heat transfer coefficient K, and a temperature difference dT between the first fluid and the second fluid have a relationship represented by Expression (1).

[Math. 1]

Q=A·K·dT  (1)

Further, the heat transfer coefficient K may be expressed by Expression (2).

$\begin{matrix} \left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack & \; \\ {K = \frac{\lambda \; L}{\left\{ {{1\text{/}\left( {\alpha_{1} \cdot d_{1}} \right)} + {1\text{/}\left( {\alpha_{2} \cdot d_{2}} \right)} + {1\text{/}{\left( {2 \cdot \lambda} \right) \cdot {\ln \left( {d_{10}\text{/}d_{11}} \right)}}} + R} \right\}}} & (2) \end{matrix}$

Note that, the meaning of each symbol is as follows. α1: heat transfer coefficient of fluid 1, d1: hydraulic diameter of flow path 1, α2: heat transfer coefficient of flow path 2, d2: hydraulic diameter of flow path 2, λ: thermal conductivity of inner pipe, dio: outer diameter of inner pipe, doi: inner diameter of inner pipe, R: thermal resistance

The above-mentioned heat transfer area increasing pipe 11 functions as a fin when held in contact with the inner pipe 5, and hence the heat transfer area can be increased to increase a heat exchange amount between the first fluid and the second fluid.

In this case, a flowing state of a refrigerant in a case where a gas-liquid two-phase flow flows in the second flow path 23 is described referring to FIG. 3 and also to FIG. 4. FIG. 3 is a view in the same manner as in FIG. 2, for illustrating the second flow path in an enlarged manner. FIG. 4 is a view for illustrating a part of FIG. 3, in which the outer pipe, the heat transfer area increasing pipe, and the inner pipe are separated from each other for the sake of illustration. In this case, in general, in the two-phase flow, a liquid refrigerant having a higher heat transfer coefficient is held in close contact with a pipe wall, and a gas refrigerant having a lower heat transfer coefficient flows in a portion away from the pipe wall. That is, the liquid refrigerant concentrates on wall surfaces indicated by reference symbols 3 b, 5 a, 17 b, 19 a, 21 a, and 21 b in FIG. 3.

In view of the above, the present invention sets a groove non-forming range and a groove forming candidate range as described below. A non-groove surface is formed in the groove non-forming range, and grooves extending along a flow direction are formed in at least a part or the entirety of the groove forming candidate range. The first embodiment is an example of a case where the grooves are formed in the entirety of the groove forming candidate range.

The groove non-forming range and the groove forming candidate range are described in detail. Specifically, the groove non-forming range corresponds to, in an inner surface of the heat transfer area increasing pipe 11, an inner surface of a part of the heat transfer area increasing pipe 11, which is held in close contact with the inner surface 3 b of the outer pipe 3 (inner surface 17 b of the outer close-contact portion 17). In addition, the groove non-forming range also corresponds to a part of the inner surface 3 b of the outer pipe 3, which defines the second flow path 23 in cooperation with an outer surface of the heat transfer area increasing pipe 11. Grooves 25 described later are not formed in each of those groove non-forming ranges.

Further, the groove forming candidate range is formed by a part excluding the above-mentioned groove non-forming range (inner surface 17 b of the outer close-contact portion 17) from a part of the inner surface of the heat transfer area increasing pipe 11, which defines the second flow path 23 in cooperation with the outer surface 5 a of the inner pipe 5 (inner surfaces 21 b of the continuous portions 21), a part of the outer surface of the heat transfer area increasing pipe 11, which defines the second flow path 23 in cooperation with the inner surface 3 b of the outer pipe 3 (outer surfaces 21 a of the continuous portions 21 and the outer surface 19 a of the inner close-contact portion 19), and a part of the outer surface 5 a of the inner pipe 5, which defines the second flow path 23 in cooperation with the inner surface of the heat transfer area increasing pipe 11.

In the first embodiment, the grooves are not formed in the groove non-forming range as described above, and the grooves are formed in the entirety of the groove forming candidate range. More specific description is given below. The grooves 25 are formed in each of the part of the outer surface 5 a of the inner pipe 5, which defines the second flow path 23 in cooperation with the outer close-contact portion 17 and the pair of the continuous portions 21, the outer surface 19 a of the inner close-contact portion 19 of the heat transfer area increasing pipe 11, and the outer surfaces 21 a and the inner surfaces 21 b of the continuous portions 21. Further, the non-groove surface is formed on each of the inner surface 17 b of the outer close-contact portion 17 and the part of the inner surface 3 b of the outer pipe 3, which defines the second flow path 23 in cooperation with the inner close-contact portion 19 and the pair of the continuous portions 21. Note that, although not particularly limited as the present invention, in the first embodiment, the non-groove surface is formed on each of the outer surface 17 a of the outer close-contact portion 17 of the heat transfer area increasing pipe 11 and a part of the inner surface 3 b of the outer pipe 3, which is held in close contact with the outer surface 17 a. In addition, the non-groove surface is formed on each of the inner surface 19 b of the inner close-contact portion 19 and a part of the outer surface 5 a of the inner pipe 5, which is held in close contact with the inner surface 19 b.

The grooves 25 are formed in a mode of extending along the flow direction so as to allow the refrigerant to flow smoothly in the flow direction. Note that, the grooves are schematically illustrated in FIG. 3 and FIG. 4, and further, in FIG. 2, the illustration of the grooves is omitted for the sake of clarity of illustration.

Note that, it is conceivable that the heat transfer area increasing pipe 11 is formed through press forming or a drawing process. Therefore, in order to simplify the process, the grooves 25 are simultaneously formed at the time of the press forming or the drawing process. Further, the heat transfer area increasing pipe 11 having the grooves 25 formed therein is inserted to the annular region 9 between the outer pipe 3 and the inner pipe 5, and the outer pipe 3 is reduced in diameter or the inner pipe 5 is increased in diameter. In this manner, the heat transfer area increasing pipe 11 is supported by the outer pipe 3 and the inner pipe 5.

Alternatively, as a method of further reliably holding the inner pipe 5 and the outer pipe 3 in close contact with the heat transfer area increasing pipe 11, a mode of joining respective contact surfaces through brazing is also preferred. Specifically, after the heat transfer area increasing pipe 11 is assembled to the outer pipe 3 and the inner pipe 5, a brazing material is applied to the contact surfaces, and the brazing material is melted through brazing in a furnace or the like. In this manner, the contact surfaces may be brazed to each other. Further, in a case where the brazing material is difficult to be applied after the heat transfer area increasing pipe 11 is assembled to the inner pipe 5 and the outer pipe 3, the brazing may be performed using a cladding material having the brazing material applied thereto in advance as the heat transfer area increasing pipe 11.

According to the double-pipe heat exchanger 1 configured as described above, the following excellent advantages can be obtained. Of the parts that define the second flow path 23, a predetermined part of the outer surface 5 a of the inner pipe 5 and the outer surface 19 a of the inner close-contact portion 19 are parts extremely close to the first flow path 7, and parts having highest effectiveness as heat transfer surfaces. Further, the continuous portion 21 is formed between the above-mentioned first-mode part and second-mode part of the second flow path 23, and inner and outer surfaces of the continuous portion 21 are effective heat transfer surfaces when the continuous portion 21 exchanges heat between the second fluids of the first-mode part and the second-mode part (internal relationship of the second flow path 23) by exerting an effect of the fin. Therefore, with the grooves 25 formed as described above, the liquid refrigerant can actively be gathered on the inner and outer surfaces of the continuous portion 21, and the predetermined part of the outer surface 5 a of the inner pipe 5 and the outer surface 19 a of the inner close-contact portion 19 that is held in close contact with the inner pipe 5, which are closer to the first flow path 7. Further, in addition to the above, the non-groove surface is formed on each of a predetermined part of the inner surface 3 b of the outer pipe 3 and the inner surface 17 b of the outer close-contact portion 17, which are farther from the first flow path 7 and have lower effectiveness as heat transfer surfaces. With this, the liquid refrigerant is less likely to gather on the non-groove surface relative to the predetermined part of the outer surface 5 a or the outer surface 19 a. As a countereffect, the liquid refrigerant is assisted so as to gather on the predetermined part of the outer surface 5 a, the outer surface 19 a, and the inner and outer surfaces of the continuous portion 21. That is, the following matter is prevented. Specifically, a large amount of the liquid refrigerant having the higher heat transfer coefficient is supplied also to each of the predetermined part of the inner surface 3 b of the outer pipe 3 and the inner surface 17 b of the outer close-contact portion 17, which have the lower effectiveness as the heat transfer surfaces, to thereby correspondingly reduce a supply amount of the liquid refrigerant to each of the predetermined part of the outer surface 5 a, the outer surface 19 a, and the inner and outer surfaces of the continuous portion 21, which have the higher effectiveness as the heat transfer surfaces. As described above, according to this embodiment, even in the case where the gas-liquid two-phase flow flows in the second flow path, heat exchange performance can be enhanced by effectively utilizing the heat transfer surfaces.

In addition, in the first embodiment, the non-groove surface is formed on each of the outer surface 17 a of the outer close-contact portion 17 of the heat transfer area increasing pipe 11 and the part of the inner surface 3 b of the outer pipe 3, which is held in close contact with the outer surface 17 a. Similarly, the non-groove surface is formed on each of the inner surface 19 b of the inner close-contact portion 19 and the part of the outer surface 5 a of the inner pipe 5, which is held in close contact with the inner surface 19 b. Thus, close contact performance between the heat transfer area increasing pipe 11 and each of the inner pipe 5 and the outer pipe 3 can be maintained high. Not only that, particularly because the close contact performance between the inner pipe 5 and the heat transfer area increasing pipe 11 is high, an efficiency of the thermal conduction of the heat transfer area increasing pipe 11 can be enhanced. Thus, the provision of the heat transfer area increasing pipe 11 can efficiently be utilized.

Next, Examples of a refrigeration cycle system to which the above-mentioned double-pipe heat exchanger 1 is applied are described referring to FIG. 5 to FIG. 8.

As Example 1 of the refrigeration cycle system, a refrigeration cycle system 101 illustrated in FIG. 5 includes, as circuit main components, a compressor 103, a condenser 105, an expansion valve 107, an evaporator 109, and the above-mentioned double-pipe heat exchanger 1. In the double-pipe heat exchanger 1, heat is exchanged between a high-pressure liquid refrigerant (second fluid) from an outlet of the condenser 105 (before flowing into an inlet of the expansion valve 107), and a low-pressure gas refrigerant (first fluid) from an outlet of the evaporator 109 (before flowing into an inlet of the compressor 103). With the use of the double-pipe heat exchanger 1 as described above, an inlet temperature of the condenser 105 is increased. Thus, performance in heating can be enhanced to enhance COP (value obtained by dividing the performance by an input), or the liquid refrigerant can be prevented from returning to the compressor.

Next, as Example 2 of the refrigeration cycle system, a refrigeration cycle system 201 illustrated in FIG. 6 includes, as circuit main components, the compressor 103, the condenser 105, a first expansion valve 207 a, a second expansion valve 207 b, the evaporator 109, and the above-mentioned double-pipe heat exchanger 1. The compressor 103, the condenser 105, the first expansion valve 207 a, and the evaporator 109 construct a basic refrigeration cycle circuit similarly to the case of Example 1. A bypass passage 211 is further formed in the refrigeration cycle system 201. The bypass passage 211 is connected at a first connecting point 213 a to a part from the outlet of the condenser 105 to an inlet of the first expansion valve 207 a, and is connected at a second connecting point 213 b to a part from the outlet of the evaporator 109 to the inlet of the compressor 103. The second expansion valve 207 b is arranged in the bypass passage 211.

In the double-pipe heat exchanger 1, heat is exchanged between a high-pressure liquid refrigerant (first fluid) from the outlet of the condenser 105 (before reaching the first connecting point 213 a), and an intermediate-pressure gas-liquid two-phase refrigerant (second fluid) from the outlet of the second expansion valve 207 b of the bypass passage 211. The intermediate-pressure gas refrigerant after undergoing the heat exchange in the double-pipe heat exchanger 1 is sucked into the compressor 103. With the use of the double-pipe heat exchanger as described above, a refrigerant circulation amount in a downstream part with respect to the first expansion valve 207 a can be reduced to reduce pressure loss, thereby enhancing the COP.

Next, as Example 3 of the refrigeration cycle system, a refrigeration cycle system 301 illustrated in FIG. 7 includes, as circuit main components, a compressor 303, the condenser 105, the first expansion valve 207 a, the second expansion valve 207 b, the evaporator 109, and the above-mentioned double-pipe heat exchanger 1. The compressor 303, the condenser 105, the first expansion valve 207 a, and the evaporator 109 construct a basic refrigeration cycle circuit similarly to the case of Example 1.

In the double-pipe heat exchanger 1, heat is exchanged between a high-pressure liquid refrigerant (first fluid) from the outlet of the condenser 105 (before reaching the first connecting point 213 a), and an intermediate-pressure gas-liquid two-phase refrigerant (second fluid) from the outlet of the second expansion valve 207 b of the bypass passage 211. Further, the intermediate-pressure gas refrigerant after undergoing the heat exchange in the double-pipe heat exchanger 1 is caused to bypass into the middle of a compressing part of the compressor 303. With the use of the double-pipe heat exchanger as described above, a refrigerant circulation amount in the downstream part with respect to the first expansion valve 207 a can be reduced, and a compressing process can be performed in a plurality of stages to reduce an input to the compressor, thereby enhancing the COP.

In addition, a refrigeration cycle system 401 illustrated in FIG. 8 uses the double-pipe heat exchanger 1 as a condenser itself of the basic refrigeration cycle circuit. The refrigeration cycle system 401 is an example of a system of exchanging heat between the refrigerant (second fluid) in the general condenser of the refrigeration cycle circuit and a fluid (first fluid), such as water or brine, fed by a pump 415 in the double-pipe heat exchanger 1, to thereby supply hot water.

Second Embodiment

Next, a second embodiment of the present invention is described. FIG. 9 is a view according to the second embodiment of the present invention, for illustrating in the same manner as in FIG. 3. The second embodiment is the same as the above-mentioned first embodiment except for a part described below. Further, the second embodiment is the same as the first embodiment also in that the second embodiment may be carried out by the refrigeration cycle system of FIG. 5 to FIG. 8.

A double-pipe heat exchanger 51 is an example in which the grooves 25 extending along the flow direction are formed in at least a part of the groove forming candidate range. That is, in the second embodiment, as illustrated in FIG. 9, the grooves 25 are only formed in the inner and outer surfaces of the continuous portion 21, in the groove forming candidate range that corresponds to the above-mentioned predetermined part of the outer surface 5 a of the inner pipe 5, the outer surface 19 a of the inner close-contact portion 19, and the inner and outer surfaces of the continuous portion 21. In such a second embodiment, similarly to the first embodiment, the liquid refrigerant can efficiently be gathered on the inner and outer surfaces of the continuous portion 21, which have the higher effectiveness as the heat transfer surfaces. Even when the gas-liquid two-phase flow flows in the second flow path, the heat exchange performance can be enhanced by effectively utilizing the heat transfer surfaces.

Third Embodiment

Next, a third embodiment of the present invention is described. FIG. 10 is a view according to the third embodiment of the present invention, for illustrating in the same manner as in FIG. 3. The third embodiment is the same as the above-mentioned first embodiment except for a part described below. Further, the third embodiment is the same as the first embodiment also in that the third embodiment may be carried out by the refrigeration cycle system of FIG. 5 to FIG. 8.

A double-pipe heat exchanger 61 is also an example in which the grooves 25 extending along the flow direction are formed in at least a part of the groove forming candidate range. In the third embodiment, as illustrated in FIG. 10, the grooves 25 are only formed in the above-mentioned predetermined part of the outer surface 5 a of the inner pipe 5 and the outer surface 19 a of the inner close-contact portion 19, in the groove forming candidate range that corresponds to the above-mentioned predetermined part of the outer surface 5 a of the inner pipe 5, the outer surface 19 a of the inner close-contact portion 19, and the inner and outer surfaces of the continuous portion 21. In such a third embodiment, similarly to the first embodiment, even when the gas-liquid two-phase flow flows in the second flow path, the heat exchange performance can be enhanced by effectively utilizing the heat transfer surfaces.

Although the details of the present invention are specifically described above with reference to the preferred embodiments, it is apparent that persons skilled in the art may adopt various modifications based on the basic technical concepts and teachings of the present invention.

For example, the above-mentioned first embodiment may be modified so that the grooves 25 are formed also in the outer surface 17 a of the outer close-contact portion 17 of the heat transfer area increasing pipe 11. With such a modification, the grooves 25 are formed in the entire outer surface of the heat transfer area increasing pipe 11 as a uniform process. Thus, facilitation of manufacture due to the uniformity of the process can be achieved. Further, even with such a modification, the outer surface 17 a of the outer close-contact portion 17 of the heat transfer area increasing pipe 11, which is held in close contact with the outer pipe 3, is less important as the heat transfer surface. The modification does not lower the effectiveness of the present invention from the viewpoint of utilization of the heat transfer surface. That is, easiness of the manufacture can be enhanced while suitably maintaining effective utility of the heat transfer surface of the present invention.

REFERENCE SIGNS LIST

-   -   1, 51, 61 double-pipe heat exchanger, 3 outer pipe, 5 inner         pipe, 7 first flow path, 9 annular region, 11 heat transfer area         increasing pipe, 23 second flow path, 25 groove, 101, 201, 301,         401 refrigeration cycle system 

1. A double-pipe heat exchanger, comprising: an outer pipe; an inner pipe inserted to an inner side of the outer pipe, the inner pipe forming an annular region between the outer pipe and the inner pipe, and forming a first flow path in an inner side thereof; and a heat transfer area increasing pipe arranged on the inner side of the outer pipe and an outer side of the inner pipe, the heat transfer area increasing pipe having projections and depressions in a radial direction, and forming a second flow path in the annular region, wherein a non-groove surface is set in each of, in an inner surface of a part of the heat transfer area increasing pipe, which is held in close contact with an inner surface of the outer pipe, and a part of the inner surface of the outer pipe, which defines the second flow path in cooperation with an outer surface of the heat transfer area increasing pipe, and wherein grooves are formed in at least a part or an entirety of a part excluding the non-groove surface from wall surface forming the second flow path.
 2. A double-pipe heat exchanger according to claim 1, wherein the non-groove surface is formed on each of a part of the inner surface of the outer pipe, which is held in close contact with the outer surface of the heat transfer area increasing pipe, a part of the outer surface of the heat transfer area increasing pipe, which is held in close contact with the inner surface of the outer pipe, a part of the outer surface of the inner pipe, which is held in close contact with the inner surface of the heat transfer area increasing pipe, and a part of the inner surface of the heat transfer area increasing pipe, which is held in close contact with the outer surface of the inner pipe.
 3. A double-pipe heat exchanger according to claim 1, wherein, after the grooves are formed in the heat transfer area increasing pipe, the heat transfer area increasing pipe is inserted to the annular region between the outer pipe and the inner pipe, and the outer pipe is reduced in diameter or the inner pipe is increased in diameter so that the heat transfer area increasing pipe is supported by the outer pipe and the inner pipe.
 4. A double-pipe heat exchanger according to claim 1, wherein the inner pipe and the outer pipe are brazed to the heat transfer area increasing pipe.
 5. A double-pipe heat exchanger according to claim 4, wherein the heat transfer area increasing pipe comprises a cladding material having a brazing material covered on a surface thereof.
 6. A refrigeration cycle system, comprising the double-pipe heat exchanger of claim 1, wherein heat is exchanged between refrigerants in the double-pipe heat exchanger.
 7. A refrigeration cycle system, comprising the double-pipe heat exchanger of claim 1, wherein heat is exchanged between a refrigerant and water or between a refrigerant and brine in the double-pipe heat exchanger.
 8. A double-pipe heat exchanger according to claim 1, wherein a groove forming candidate range comprises a part excluding the groove non-forming range from a part of the inner surface of the heat transfer area increasing pipe, which defines the second flow path in cooperation with an outer surface of the inner pipe, a part of the outer surface of the heat transfer area increasing pipe, which defines the second flow path in cooperation with the inner surface of the outer pipe, and a part of the outer surface of the inner pipe, which defines the second flow path in cooperation with the inner surface of the heat transfer area increasing pipe.
 9. A double-pipe heat exchanger according to claim 1, wherein grooves extend along a flow direction. 